1. Field of the Invention
This invention relates to an audio system, its reproducing apparatus, and its recording medium, and more particularly to an audio system that easily maintains compatibility with a surround system when in a system that transmits or records (hereinafter, transmits) surround audio using a plurality of transmission channels, the plurality of channels are used for special purposes (e.g., such as karaoke) which are not limited to surround, a reproducing apparatus for reproducing audio signals from the system, and a recording medium made according to an allocating method complying with the standard for surround.
2. Description of the Related Art
A conventional surround system, for example, a Dolby surround system AC-3 proposed by Dolby Laboratories, is such that when 5-channel surround audio signals are recorded on a recording medium such as an optical disk, the audio signals are reproduced by a reproducing apparatus compatible with the surround system at three speakers in front of the listener and two speakers behind the same listener. Such conventional systems use this speaker arrangement to form a sound field around the listener giving the feeling of being at a live performance. In addition, the surround system can reproduce audio signals suitable for a 2-channel audio system. The 2-channel audio system converts 5-channel audio signals into 2-channel audio signals using a process known as down-mixing. Based on the 2-channel audio signals, stereo sound is reproduced at the two speakers positioned in front of the listener.
Conventional systems that provide surround reproduction and stereo reproduction of the 5-channel surround audio signals are shown in FIGS.1A and 1B, respectively. In surround reproduction, 5-channel audio input signals L, C, R, LS, and RS are transmitted to speakers 8L, 8C, 8R, 8LS, and 8RS at the front left, the front center, and the front right, the back left and the back right, respectively, as shown in FIG. 1A. The respective speakers reproduce sound based on the signal input thereto. As described above, in the reproducing system with 5-channel speakers, the sound volume can be adjusted channel by channel. In addition, reproduction can be stopped in any or all channels.
On the other hand, in stereo reproduction, the five input signals L, C, R, LS, and RS undergo down-mixing at a down mixer Dmix. During down-mixing, the input signals arc converted into 2-channel audio output signals Lo and Ro, which are then supplied to the front left and front right speakers 8L and 8R. Each of the two speakers 8L and 8R reproduce sound based on the signal input thereto. The down mixer Dmix includes a circuit that down-mixes 5-channel audio input signals L, C, R, LS, and RS as described hereinafter.
The 2-channel audio output signals Lo and Ro, which are obtained by down-mixing and supplied to the front left and front right speakers 8L and 8R, are normally expressed as: EQU Lo=L+0.7C+0.7LS EQU Ro=R+0.7C+0.7RS
When 5-channel audio signals are converted into 2-channel audio signals, simple down-mixing may provide unnatural 2-channel reproduction depending on the nature of the 5-channel audio signals. For instance, there may be a case where absence of the surround-channel audio signals LS and RS results in natural reproduction. For this reason, the conventional down mixer Dmix uses a circuit controlled by a control signal Cont to selectively turn on and off the addition of the surround channel audio signals LS and RS, as shown in FIG. 2. To prevent a sense of incongruity from being introduced into the sound field in reproduction, the control signal Cont is not manipulatable by the user. As a rule, the user cannot enter the control signal Cont into the down mixer arbitrarily. Generally, the control signal is determined on the transmission side or the recording side and recorded on an optical disk or a like recording medium. When down-mixing is performed, the audio signals transmitted via a plurality of transmitting of channels are reproduced at output channels, where the number of output channels is smaller than the number of transmitting channels. During conventional down-mixing the user cannot change the conversion coefficients including the mixing ratio.
In recent years, in Japan and many other foreign countries, karaoke systems have become very popular, where the user himself or herself sings with background music. Although various karaoke systems have been proposed and those with various functions have been developed, a multi-functional karaoke system that can sufficiently comply with the demands of users has not yet been realized. Similarly, there have been various special demands in systems such as karaoke, for example, when of special reproduction uses, such as a rendering of a particular musical instrument may be reproduced in addition to karaoke.
In systems such as karaoke, to comply with the demands of the users appropriately, it has been necessary to prepare as many audio programs as there are demands and to pre-record data for each audio program on a recording medium. Such systems are problematic since they require a tremendous recording capacity. To reduce memory requirements, a surround sound system has been considered. However, since conventional surround sound systems have not taken into account special applications including karaoke, it is difficult to simply apply such special applications to a surround sound system. Thus, in conventional surround sound systems used for special applications such as karaoke, some improvements are needed. Specifically, when down-mixing is conducted, the audio signals transmitted via a plurality of channels are reproduced at a number of output channels fewer than the number of transmission channels. Because the user cannot change the conversion coefficients including the mixing radio once conventional downmixing is effected, it is necessary to allocate channels, taking into consideration how to handle both those audio signals normally reproduced and those audio signals not normally reproduced. As described above, to provide special applications including karaoke which comply with the various demands of the users, it is necessary to combine a conventional surround sound system with a special system (i.e., a karaoke system) and it is necessary to consider the compatibility between them.